1. Field of the Invention
The present invention relates to communication equipment and, more specifically, to equipment for wireless local area networks (WLANs).
2. Description of the Related Art
IEEE Standard 802.11, the teachings of which are incorporated herein by reference, has emerged as a prevailing technology for broadband access in WLAN systems and is regarded by many as a wireless version of Ethernet. The 802.11 medium access control (MAC) specifications provide that a wireless station (STA) may be in one of two power states: awake state and doze state. In the awake state, the STA is fully powered and is able to transmit and receive frames. In contrast, in the doze state, the STA consumes very low power and is not able to transmit or receive. The manner in which an STA transitions between these two states is determined by the STA power management mode. The 802.11 MAC specifications define two power management modes: active mode and power save (PS) mode. In active mode, the STA is always awake and therefore consumes substantial power. In PS mode, the STA is in the awake state only for relatively short periods of time while spending the remaining time in the doze state, which significantly reduces the amount of consumed power.
According to the 802.11 standard, a WLAN system having one or more STAs functioning in PS mode may operate as follows. The access point (AP) of the WLAN does not arbitrarily transmit frames to said STAs, but buffers the frames and transmits them at designated times. The AP identifies the STAs, for which the AP currently has frames, in a traffic indication map (TIM) provided with a beacon. With knowledge of beacon schedule, each STA functioning in PS mode awakes for beacons and determines by interpreting the TIM whether the AP has a buffered frame for that STA. Upon determining that the AP currently has a buffered frame, the STA transmits a PS-Poll frame indicating that it is awake and is ready to receive. In response, the AP may either transmit the buffered frame immediately or acknowledge receipt of the PS-Poll frame and transmit the buffered frame at a later time. The STA remains in the awake state to await the frame transmission.
One problem with the above-described operating method is that it typically creates a transmission overhead of one PS-Poll frame per each buffered frame. As a result, an application employing relatively small data frames exchanged with relatively high periodicity, e.g., interactive voice over WLAN, will create a disadvantageously large transmission overhead. Another problem is that the delivery of buffered frames tends to be concentrated around beacons, which creates congestion, thereby increasing the number of collisions and reducing effective channel capacity.